Digital circuits rely heavily on having a constant source of voltage available. Unlike, for example, a flashlight that glows more dimly as battery voltage diminishes, a processor does not just slow down its processing when voltage diminishes. Once voltage is outside an acceptable range, it simply ceases to operate.
A power converter ensures the availability of a suitable voltage. Many power converters have switched-capacitor circuits or regulators that participate in maintaining a steady voltage. These circuits often rely on switches that open and close quickly and often.
A switch is often implemented as a MOSFET. A MOSFET has a channel that extends between a source and a drain. Assuming that a suitable voltage has been applied between the source and the drain, closing the switch causes current to flow through the channel between the source and the drain. Opening the switch halts this flow.
Opening and closing the switch involves controlling the properties of this channel. To close the switch, this channel must be made to support conduction. To open the switch, this channel must be made to inhibit conduction.
The transition between a channel that supports conduction and one that inhibits conduction is carried out by controlling a gate-drive voltage at a gate terminal of the MOSFET. To create a gate-drive voltage, charge is made to flow into the gate terminal so that it collects on a suitable surface and establishes an electric field in the channel. When the electric field is no longer needed, the charge is made to flow back out of the gate terminal. The presence or absence of this electric field is what opens and closes the switch.
The establishment of this electric field is not instantaneous. It takes some time for charge to flow into the gate. There is also a delay between the decision to change the state of the switch and the execution of that decision at the gate terminal.
For example, in order to change the state of a switch, a controller sends a signal to the switch. The signal makes its way through a switch driver, which then ultimately controls the voltage at the gate terminal. These delays place an upper limit on how fast the switch can be made to turn on and off. This upper limit, in turn, places limits on what the power converter can do.